I just got an estimate for a Doyle sail cradle, which is similar to a stack pack. The estimate is for $100/ft of boom (17' in my case ). Does this seem high, or about right? That is the uninstalled price.

The $100.00 a ft. for the stack packs or "Lazy Cradles" is about right.
However, Calvert Sails in Florida is offering a promotion on these at $55.00 a ft.
These include choice of sunbrella color, round battens on both sides along the top, heavy duty #10 zipper, and full lazy jack package with lines and blocks.